Numerous multimedia services have emerged along with rapid development of mobile communication and the Internet, and in some of the application services such as Video-On-Demand, television broadcast, video conferencing, online education, and interactive games, a plurality of users need to receive the same data simultaneously. These mobile multimedia services are characterized by a large amount of data, a long duration, etc., as compared with general services. For effective utilization of mobile network resources, the 3rd Generation Partnership Project (3GPP) of Mobile Communication Standardization organization has proposed the Multimedia Broadcast/Multicast Service (MBMS). The MBMS refers to a point-to-multipoint service for transmitting data to a plurality of users from one data source, which enables sharing of network resources including mobile core network and access network resources, particularly air interface resources. The MBMS can accomplish not only the multicast and broadcast of plain-text and low rate messages but also the multicast and broadcast of high rate multimedia services, e.g., a mobile TV service.
The MBMS suffered from a low spectral efficiency which is typically at 0.2˜0.02 bit/Hz/s in the original 3GPP Release 6, and therefore the 3GPP commenced on a study on the Enhanced Multimedia Broadcast/Multicast Service (E-MBMS) in the Long Term Evolution (LTE) architecture, with the Single Frequency Network (SFN) transmission technology in the existing industry terrestrial broadcasting standard being introduced into the E-MBMS.
The SFN transmission technology refers to that the same MBMS data is transmitted concurrently in all cells of an SFN network using the same physical resources, e.g., the same frequency, the same code channel, the same scrambling code, and the same channel estimation code, so that even a user equipment in an edge zone of the cell may receive available signals from different cells. The user equipment can not only integrate energies of all the received available signals directly in an air interface but also obtain diversity gains from different paths. FIG. 1 illustrates a topology diagram of the structure of an SFN network in which each cell is consisted of three sectors sharing a station address, that is, these three sectors have the same base station address. A specific current implementation solution of the SFN transmission technology in the 3GPP E-MBMS is described below taking the structure of the SFN network as an example: when a core network initiates a broadcast service, a Radio Network Controller (RNC) allocates collectively for all sectors in the SFN network the same temporal resources, frequency resources, scrambling code and channel estimation code used for the broadcast service, and the temporal resources, the frequency resources, the scrambling code and the channel estimation code are also used by a user equipment in a cell to receive the broadcast service, that is, as long as a signal from a sector in the SFN network falls within the window of a multi-path receiver of the user equipment, the user equipment may integrate energies of all such signals falling within the reception window directly to thereby improve greatly reception performance of the broadcast service.
A method for implementing E-MBMS transmission in an existing SFN network of a Time Division-Synchronous Code Division Multiple Address (TD-SCDMA) system is described below on the basis of resource allocation in the above SFN transmission technology and the implementation thereof Referring to FIG. 2, when a core network initiates a broadcast service and transmits the broadcast service to multiple sectors, a RNC allocates collectively for base stations of the multiple sectors the same broadcast service resources including a transmission time, a transmission frequency, and a scrambling code and a channel estimation (midamble) code used for the broadcast service, and notifies the base stations (Node Bs) and user equipments of information of the broadcast service resources through signaling; the base stations of the respective sectors form broadcast service signals from broadcast service data according to these specific codes and transmit the broadcast service signals using the service recourses allocated by the RNC; after receiving the same broadcast service signals transmitted concurrently from the multiple cells at corresponding recourse locations, the user equipment performs channel estimation using the specified midamble code and descrambles the data using the specified scrambling code to thereby derive the desired broadcast service data. The method generally includes the following steps:
Step 1: The core network initiates a broadcast service and notifies the RNC of the broadcast service, and the RNC determines those sectors to which the broadcast service is to be transmitted.
Step 2: The RNC allocates collectively for base stations of those sectors the same broadcast service resources including the same transmission time, transmission frequency, transmission code channel, and scrambling code and midamble code used for the broadcast service.
The codes specified here are different from scrambling codes and midamble codes adopted for the existing non-broadcast services. Groups of codes relatively correlative with the existing scrambling codes and midamble codes may be designated in advance for broadcast services and dedicated to scrambling codes and midamble codes of the broadcast services, to thereby form a table of broadcast service code groups, which is stored in the RNC, user equipments and base stations, and upon allocation of resources for a specific broadcast service, the RNC selects a pair of codes from the table and informs the base stations and the user equipments about a serial number corresponding to the code group, thereby reducing a signaling load.
Step 3: Upon transmission of the broadcast service, the base station forms broadcast service signals from relevant broadcast service data according to the scrambling code and the midamble code allocated by the RNC and transmits the broadcast service signals using the service resources allocated by the RNC.
Step 4: The user equipment receives the signals over corresponding resources according to the resource allocation information transmitted from the RNC. Signals transmitted from multiple cells may be received by the user equipment, and at this time the user equipment performs channel estimation according to the specified midamble code to derive an overall condition of channels from the multiple cells to the user equipment, and then de-spreads the data based upon a result of the channel estimation and subsequently descrambles the de-spreaded data using the specified scrambling code to derive the desired broadcast service data.
Some drawbacks are still to be overcome in the existing E-MBMS transmission method despite its significant improvement over the traditional MBMS transmission technology. For adjacent sectors in an SFN network, the phenomenon of fast fading of a signal may likely occur in adjacent edge zones of the sectors, and this phenomenon refers to that when the same signals are transmitted from two base stations, if the signals arrive at a receiver of a user equipment concurrently but with opposite phases, then energy of the received signals may be superposed totally inversely, resulting in significant rapid attenuation and hence a considerably degraded quality of the received signals. This phenomenon may be more appreciable in adjacent edge zones of adjacent sectors in the same cell due to the extremely similar channel environment in the adjacent edge zones of the adjacent sectors in the same cell. As illustratively shown in FIG. 3, two adjacent sectors in the same cell, e.g., sectors 1 and 0, 1 and 2, and 0 and 2, share a common station address and hence the channel environments of the adjacent edge zones of the two adjacent sectors are extremely similar, so that service signals transmitted from two base stations of the adjacent sectors may likely arrive at the edge zones of the two adjacent sectors simultaneously. Further, the two service signals from the adjacent sectors may likely have identical or opposite phases due to their relatively similar fading features. In the case that the two service signals have opposite phases, energy of the two signals arriving at the user equipment may be superposed totally inversely, resulting in significant rapid attenuation and hence a considerably degraded quality of the received signals, which has also been demonstrated from simulation results.